Phosphor coatings on the interior surfaces of fluorescent lamps are achieved by contacting the glass surface of the lamp envelope with a suspension of the phosphors in such a manner as to yield a consistent, uniform coating of the entire surface. The excess suspension is drained away under force of gravity and the wet coating is dried by passing hot air through the lamp. The dried coating is then baked at an elevated temperature in air to decompose the organic binder to obtain the finished coating. The texture of the phosphor coating in the finished lamp is very sensitive to the degree of dispersion of the phosphors in the coating suspension. Flocculation of the phosphor particles leads to an unacceptable lamp texture. Thus, the coating suspension has to be adjusted to prevent this undesirable phenomenon.
In aqueous suspensions of phosphors, the stability of the suspension depends on a balance between electrostatic repulsive forces and van der Waals attractive forces. Flocculation may occur between particles of the same phosphor (intra-particle), or, as in the case of phosphor blends, it may occur between particles of dissimilar phosphors (inter-particle). Phosphor particles in an aqueous suspension develop a surface charge which is dependent upon the pH of the suspension and the Isoelectric Point (IEP) of the phosphor. The IEP is defined as the pH at which zeta potential is zero. If the pH of the suspension is above the IEP of a phosphor, then the phosphor particles are negatively charged, if the pH is lower than the IEP, then the phosphor particles are positively charged.
In an aqueous suspension of a multi-component phosphor blend, the phosphors can have widely different IEP's. In such a situation, flocculation will occur unless the pH of the solution is such that all of the phosphor particles have surface charges which have the same polarity and which are of sufficient magnitude to overcome the van der Waals attractions. In other words, stability is achieved by ensuring that there is a net electrostatic repulsive force between the phosphor particles in the suspension.
In U.S. Pat. No. 5,234,710 to Dutta et al., flocculation is inhibited in an aqueous coating suspension for a suntanning lamp by adjusting the pH of the aqueous suspension to a value which is outside of the pH range defined by the IEP's of the individual phosphors in the two-component blend. However, this approach is not applicable to all types of phosphor systems. In particular, multi-component phosphor blends which contain phosphors of widely varying IEP's can require adjusting the pH of the suspension either to very low pH, highly acidic conditions or very high pH, highly basic conditions. Such pH extremes can cause the dissolution of phosphors in the suspension and pose environmental and safety problems for the manufacturer. Thus, it would be advantageous to provide a method which inhibited flocculation in these systems without having to use suspensions having pH values lower than 7 or greater than 11. Preferably, the aqueous suspension would have a manageable pH of between 7 and 10.